glsl: pass shader stage to lower_output_reads and handle tess control

Tessellation control outputs can be read in directly without first
having been written. Accessing these will require some special logic
anyways, so just let them through.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
hack
push consumer input vertices handling down
support producer_vertices
rename gs_input_vertices to num_vertices